One of the more common constructions of optical wavelength division multiplexed (WDM) filters is a WDM filter element typically comprising a multi-layered thin film structure deposited on a transparent substrate, between two, substantially quarter-pitch graded index (GRIN) lenses. In some instances the thin film layers are coated to one of the end faces of the lens. Lenses of this type are produced under the trade name "SELFOC"; the mark is registered in Japan and owned by the Nippon Sheet and Glass Co. Ltd.
At a location along the lens, in this instance the end face, indicated as an approximately 0.25 pitch, the input beam becomes collimated. This phenomenon is further demonstrated in FIG. 1, where, two matched quarter pitch GRIN lenses are disposed in a back to back relationship. Each GRIN lens is provided with a port, which is a point or region along an end face of the lens for receiving or transmitting a beam of light.
Although these WDM filters perform their intended function, filters having a sharper, steeper more square response are at times required. This desired response can be achieved by passing a beam through multiple, similar, cascaded filters; or alternatively, and less preferred, by sandwiching two WDM filter elements together. The latter solution is however difficult to manufacture and tends not be a reliable design.
As of late, standards have evolved and have become adopted for optical systems having very narrow channel spacing in an attempt to increase bandwidth in an optical system limited by the number of available optical waveguides. For example, the International Telecommunications Union (ITU) has developed standards for 100 and 200 GHz spacing of channels, and in an attempt to conform with these standards there has been a greater requirement for narrow band optical filters that have low signal loss.
As these requirements upon optical filter makers become more demanding by having to make filters that have extremely narrow passband responses, the number of thin film layers or optical interfaces may increase. As a result, the optical thickness of the substrate and filter layers of these filters also increases and light passing therethrough is affected differently, than a beam passing through a thin optical interface. Hence, the coupling of light passing through a filter having a large optical thickness becomes more difficult.
It is therefore an object of this invention, to provide a device, which passes at least a wavelength channel of light through cascaded optical filters with little signal loss and minimal distortion of the filter wavelength as light is passed through the filter at an non-orthogonal angle with respect to the filter surface.
It is a further object of the invention to provide a novel, compact, optical device that substantially corrects for beam distortion due to passing a beam of light through an optical filter at an non-orthogonal angle with respect to the filter surface.